The invention pertains to an image-generating energy filter for electrically charged particles such as electrons and ions with at least two toroidal energy analyzers arranged in a row, where at least one energy analyzer has a diaphragm in its entrance plane and another diaphragm in its exit plane. The invention also pertains to the use of these image-generating energy filters.
The diaphragms in the entrance and exit planes can be slit diaphragms or circular diaphragms perpendicular to the associated energy-dispersive plane.
The term “energy filter” is understood to mean preferably an imaging or image-generating energy filter. The use of imaging filters is especially advantageous when the image fields being processed in parallel contain more than 100×100 pixels. The recording times are then much shorter than those of a spectrometer, which scans the sample sequentially.
Energy filters are used in, for example, photoelectron spectroscopy, which is one of the most important methods of the quantitative elementary analysis of surfaces. Measuring the energy distribution of photoelectrons with high local resolution is called spectromicroscopy. There are essentially two different methods which can be used to achieve a high degree of local resolution.
In the first variant, the sample is scanned by a focused photon beam, and the energies of the photoelectrons coming from the individual emission spots thus defined are analyzed.
In the second variant, the photon beam is focused just long enough to illuminate the visual range of the objective lens. Electron-optical means are then used to produce a magnified image of the intensity distribution of the generated photoelectrons.
To derive a map of the distribution of the elements or of the chemical bonds, the kinetic energies of the photoelectrons must be analyzed. Various techniques have been developed to accomplish this in transmission-electron microscopy. Here, too, there are essentially two different principles:
There are microscopes which use all of the electrons to generate an image. A small percentage of the electrons pass through an energy analyzer to generate a spectrum of a portion of the image. In another part of the microscope, only a narrow energy band is processed, but a complete image is transported through the energy analyzer.
The electrons are filtered by electrostatic or magnetic devices, which allow only the electrons with a certain energy to pass through. The intensity of the resulting beam reflects the concentration of a chemical component present on the surface of the sample. In this method, it is important for the local resolution not to deteriorate as the beam passes through the monochromator.
Several different energy analyzers have been developed to perform this imaging function. Because of its good transmission and energy resolution, the hemispherical analyzer has become widely accepted in commercial devices for energy analysis not requiring image quality.
The possible imaging properties of electrostatic energy analyzers were studied many years ago on the basis of analyzers with general toroidal fields (B. Wannberg, G. Engdahl, A Sköllermo: Imaging properties of electrostatic energy analyzers with toroidal fields, J. Electron Spectr. Rel. Phenomen. 9 (1976), pp. 111-127). For a toroidal potential, the radius of curvature in a first direction is different from that in a second direction perpendicular to the first. A spherical capacitor with a ratio of 1 between the radii is included as a special case in this general description. A cylindrical capacitor is curved in only one direction, and the ratio between its radii is zero. Some spectrometers make it possible to adjust the transition between the field forms in a continuously variable manner, as described, for example, in K. Jost: Novel Design of a spherical electron spectrometer, J. Phys. E.: Sci. Instr., 12, 1979, pp. 1006-1012.
An electron microscope with an energy filter comprising a spherical analyzer of hemispherical design is known from EP 0,293,924 B1. To improve the imaging quality of the energy filter, a complicated lens system is set up in front of the entrance slit so that the arriving electron beams are as close to perpendicular as possible. For electrons which start at the mean path radius r0=x0, it should be true that α0=−α1, where α0 stands for the angle at the entrance to the energy filter and α1 for the angle at the exit.
It is claimed that the entrance angles of these electrons are transferred exactly to the exit angles regardless of their energy.
To take advantage of this property, a magnified image of the sample is placed not at the entrance slit of the analyzer but rather at the focal point of a lens, which is set up in front of the slit diaphragm of the analyzer. Thus the position of the image is transformed into angles. The entrance slit diaphragm is placed on the image side of the lens at the focal point.
The exit slit of the analyzer selects the desired energy range. Another lens behind the analyzer reconstructs a now energy-filtered local image from the transmitted angle image. This can be magnified further and made visible on a screen with the help of an intensity amplifier, such as a microchannel plate.
An electron spectrometer with a similar arrangement is described in EP 0,246,841 B1. A local resolution of down to 2.5 μm is obtained with this energy analyzer of the toroidal capacitor type, which has a lens system in front and another behind.
It was overlooked, however, that the equation α1=−α0 is usually only a rough approximation. In Nucl. Instr. Methods A291 (1990), pp. 60-66, it is shown that the entrance and exit angles also depend on the entrance and exit locations. The entrance and exit angles will differ significantly from each other when the entrance and exit positions are different. It is then true that (tan α0):x0=−(tan α1):x1.
The aberrations increase with the size of the magnified image field, that is, with the possible difference between x1 and x0. The following example can illustrate the magnitude of these defects:
In the case of a visual field with a diameter of 4 mm, where, for example, x0=122 mm and x1=126 mm, and for an acceptance angle of α0=5°, we can calculate an exit angle of α1 of 5.16°. This is a 3% deviation from the incidence angle. In the case of a visual field with a radius of 100 μm, this results in an imaging error of 3 μm at the edge of the image field.
Electrons with the same entrance position but different entrance angles also have different exit positions and different exit angles according to:
      tan    ⁢                  ⁢          α      1        =      tan    ⁢                  ⁢                                        α            0                    ⁡                      (                          1              -                              2                                                      cos                    2                                    ⁢                                      α                    0                                                                        )                                    -          1                    .      
This is described in, for example, T. Sagara et al., Resolution Improvements for hemispherical energy analyzers, Rev. Sci. Instr. 71, 2000, pp. 4201-4207.
In another example, a hemispherical analyzer is used in a different operating mode. Here the potentials are selected so that the electrons travel along a hyperbolic path in a field which rises with the square of the-radius.
U.S. Pat. No. 5,185,524 describes an electrostatic analyzer with spherical mirrors. The electrons pass into the inner sphere through slits and are brought back out through the inner sphere to a focal point by an opposing field. Both the object and the image are located inside the inner sphere.
The disadvantages of this arrangement are described in Nucl. Instr. Methods 42, 1966, pp. 71-76. Large slits are present in the inner sphere at locations where the cross section of the beam is not small. Pieces of netting are attached at these points to ensure the required spherical potential. Only a portion of the field passes through the mesh, which limits the local resolving power. Each mesh opening represents a small diverging lens. Another disadvantage of using netting in the path of the beam is the production of secondary electrons, which leads to an increase in background noise and thus reduces the displayable contrast. The energy-selecting slit is located in the electrical field between the hemispheres and is therefore difficult to reach and adjust. The voltages which must be applied to the outer sphere are much higher than those required for the conventional hemispherical analyzer.
In this design, as also in the preceding one, there are inherent aberrations, which can be attributed to the merely two-fold symmetry of the instrument's construction.
DE 196 33 496 A1 describes a monochromator for electron microscopy with mirror symmetry. The design in the form of a Ω avoids second-order aberrations, and even some of the third-order aberrations disappear. One of the essential criteria for the selected design was the avoidance of an intermediate focus. The goal here is to make it possible to monochromatize a primary electron beam of small diameter and high current density. This requirement leads to a complicated mechanical solution. The design consists of eight toroidal sectors, which must be adjusted very precisely with respect to each other. The device is therefore very costly to make and very time-consuming to adjust.
A similar mirror-symmetric arrangement of monochromators is selected in EP 0,470,299 A1. This arrangement also lacks an intermediate lens, but it does have a straight connecting tube. The energy-selecting slit is located in the plane of symmetry. No provisions are made for generating images in this case, either.
An energy filter consisting of a complementary opposing pair of 90° sectors, which are arranged with respect to each other in such a way that they form an “S”, is known from U.S. Pat. No. 5,466,933. An aperture diaphragm is set up between the two sectors. With this energy filter, an image of the incoming parallel electron beam is produced at the exit from the sector arrangement.
Although this arrangement using parallel electron beams does make it possible to obtain a high-contrast image at the exit of the energy filter, the intensity present at the exit is extremely low. The intensity can be increased by allowing electrons with an entrance angle ao not equal to zero to enter as well, but then the pixels are smeared and the contrast is reduced.
WO 01/61,725 A1 describes an emission electron microscope, which contains an image-generating beam path consisting of an electron-optic imaging system, which subjects the electron beam to a parallel shift and analyzes its energy. It consists of two spherical energy analyzers with a lens inserted between them. This lens is located at the focal point of the two analyzers. An intermediate image of the sample or the angle image of the sample is placed at the center of this lens. Because the magnification of field lenses is positive, aberrations which arise on passage through the first deflector are not corrected. This document does not mention or discuss the correction of aberrations.
DE 3,014,785 A1 describes a double monochromator for charged particles, which contains a delay lens in the form of slit diaphragms between the two monochromator subunits. The monochromator operates without loss of energy resolution at higher intensities than was possible in the past. No lens which might improve the imaging properties of the system is mentioned. Slit diaphragms are also described in U.S. Pat. No. 4,742,223. The imaging properties of the system are not discussed.
U.S. Pat. No. 5,448,063 describes an image-generating, mirror-symmetric energy filter, which compensates only for 2nd and 3rd-order aberrations. This defect correction is achieved only by the use of complicated equipment, which includes additional hexapole fields.